When operating in dark murky waters, the divers or unmanned underwater vehicles cannot depend on vision as the primary sensing modality. Acoustic imaging, using SONAR, has been used successfully by submarines in these situation. Traditional SONAR does not provide high resolution imaging, except in large systems with considerable power and computational budgets.
Consider the situation where a simple hand held or head mounted acoustic "camera" with a large 2D array of ultrasonic transducers is used to image the scene in much the same way as a standard CCD camera. Subsequently, the 3D structure of the environment is reconstructed by a local processor in the users personal digital assistant (PDA). This reconstructed virtual environment can be fed directly to the divers goggles and can be superimposed on his/her visual scene, resulting in an Acoustically Enhancement Vision System (AEVS).
Our chip design aims to create a 2D array of ultrasonic transducers. The design will be the first of its kind to incorporate on chip processing. The final chip will make an ideal component for an inexpensive, low power, portable imaging system.
The ultrasonic sensors use the availability of MicroElectroMechanical (MEMS) structures available in some standard CMOS processes to realize ultrasonic transducers based on the mechanical vibration of micro-plates or membranes implemented in silicon. The plates form capacitors with the ground plane, whose values change during the vibration [Kocis, 1996]. The deviation from the nominal values depends on the amplitude of the incident wave, while the phase can be extracted from the phase of the oscillations of the capacitance value. The time of arrival of the wavefront can also be readily detected. This approach allows considerable processing electronics to be embedded with the detector element. Depending on the micro-plate/membrane design, the electronics can be placed underneath the sensing regions, effectively increasing the fill factor of the sensor to close to 100%. To facilitate the post processing required to render the 3D model of the environment and/or to assist in navigation, A/D conversion, temporal feature extraction and correlation, post sensing beam steering, region of interest localization and zooming and signal conditioning and interface standardization can be performed on chip. This system uses reflected acoustical energy from an external source and is subsequently a low power and low voltage device, making it safe for underwater usage.
Structure of the acoustic pixel
MEMS offer a unique technology for combining acoustic sensors and processing circuitry on the same chip. This capability can be exploited to realize highly integrated acoustic signal processing systems that can significantly advance the state-of-the-art of ultrasonic imaging and ranging systems. We present an example of an integrated acoustic array that can be used for imaging in murky waters. The approach allows dense arrays of sensors to be co-located with processing electronics. The processing electronics can be used to implement a variety of algorithms for improving the 3D imaging capabilities of the system. Biologically inspired processing ideas can also be implemented in our architecture. We plan to use this system for in acoustically enhanced vision system (AEVS) for underwater navigation.
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